The use of wet scrubbing processes for removing sulfur dioxide from hot flue gases has become a primary means for cleaning stack gases from power plants or other coal combusting units. Such processes usually use an aqueous lime or limestone slurry which is passed downwardly through a wet scrubbing unit to react with, and remove sulfur dioxide from, hot gases passing upwardly through the wet scrubbing unit. Especially favorable results have been commercially achieved by using aqueous lime slurries that are enriched with a magnesium component such as magnesium oxide or magnesium hydroxide, such as disclosed in U.S. Pat. Nos. 3,919,393 and 3,919,394, the contents of both patents incorporated by reference herein.
In scrubbing of hot sulfur dioxide-containing gases by magnesium enhanced lime scrubbing processes, calcium sulfite is formed which must be removed from the scrubbing system through use of a bleed stream from the recycling aqueous scrubbing medium. This bleed stream will also contain minor amounts of magnesium sulfite and chlorides. The bleed stream is normally passed to a thickener or separator where a resultant thickened sludge, or underflow, is separated and removed from the system while clarified aqueous media, or overflow, is normally returned to the aqueous scrubbing slurry in the wet scrubbing unit. The sludge removed from the wet scrubbing system contains primarily calcium sulfites and magnesium sulfites, along with various chloride salts and other impurities. The calcium sulfite sludges are difficult to dewater due to their physical properties and, when deposited into settling ponds or collection areas, require a large area and a period of time before solidification occurs.
One procedure for reducing the amount of sludge produced and discarded has been to oxidize the calcium sulfite-containing sludge so as to convert the calcium sulfite to calcium sulfate which is more easily dewatered and thus provides less volume of sludge that must be handled and used as landfill. Also, when calcium sulfate is produced as gypsum, or calcium sulfate dehydrate (CaSO.sub.4 - 2H.sub.2 O) , the gypsum can, at times, be used in various products such as industrial and building plasters, such as gypsum wallboard. The demand for gypsum is not, however, sufficiently high to absorb all of the gypsum produced by various commercial processes as well as that which would be produced if all sulfur dioxide lime scrubbing sludges were converted to gypsum. In addition, magnesium present in gypsum can have an adverse affect on the gypsum performance in conventional usage.
An especially useful form of gypsum, .alpha.-hemihydrate gypsum, or calcium sulfate .alpha.-hemihydrate (.alpha.CaSO.sub.4 .multidot.1/2H.sub.2 O) which is not normally formed in sulfur dioxide removal aqueous sludges, has specific uses which provide value over and above conventional gypsum or calcium sulfate dehydrate. The production of .alpha.-hemihydrate gypsum is generally effected by heating natural or by-product gypsum in an autoclave at elevated temperatures of above 100.degree. C. and superatmospheric pressure. The production of .alpha.-hemihydrate gypsum from gypsum obtained from power plant flue gas desulfurization processes has been proposed, for example, in U.S. Pat. Nos. 5,015,449 and 5,015,450, where moist fine grained gypsum is molded into a body and the molded body fed to an autoclave where it is subjected to a temperature in the range of 110.degree. C. to 180.degree. C. under pressure.
When gypsum (calcium sulfate dehydrate) is converted to .alpha.-hemihydrate gypsum, energy is required to drive off the excess water and provision of such energy is costly to the process.
In U.S. Pat. No. 4,069,300, a process is described for producing .alpha.-type hemihydrate calcium sulfate by oxidizing calcium sulfite in a suspension that contains at least one anionic, nonionic or amphoteric surface active agent. That process suggests that the calcium sulfite suspension used could be one produced by absorbing a waste gas containing SO.sub.2 with a slaked lime slurry. In that process, temperatures as low as 90.degree. C. are used, and no autoclave is used meaning that the process is not carried out under superatmospheric pressure, which is said to result in coarser crystals. The suspension must contain seed crystals of .alpha.-type hemihydrate of calcium sulfate which are added in an amount of between about 10 to 150 times the weight of the surface active agent used.
It is an object of the present invention to provide a method for the removal of sulfur dioxide from a hot gas stream, using an aqueous scrubbing medium containing calcium and magnesium scrubbing components and continuously produce .alpha.-hemihydrate gypsum from the scrubber effluent.
It is another object of the present invention to provide a method for the removal of sulfur dioxide from a hot gas stream, using an aqueous scrubbing medium containing calcium and magnesium scrubbing components and continuously produce .alpha.-hemihydrate gypsum from the scrubber effluent at exceptionally high conversion rates while removing magnesium sulfate therefrom.
It is yet another object of the present invention to provide a method for removal of sulfur dioxide from a hot gas stream using an aqueous scrubbing medium containing calcium and magnesium scrubbing components and continuously produce .alpha.-hemihydrate gypsum from the scrubber effluent in an energy efficient manner.
It is a further object of the present invention to provide a method for removal of sulfur dioxide from a hot gas stream using an aqueous scrubbing medium containing calcium and magnesium compounds and continuously produce .alpha.-hemihydrate gypsum while using the exothermic energy of calcium sulfite oxidation to produce heat and a portion of the pressure required to form .alpha.-hemihydrate gypsum.